A couple years ago, MS said that they were hoping to do a game every year or two now instead of every 3-5 years like with the earlier installments.

EDIT:

I found it - they said they won't necessarily do one every year (although they have with Halo 3 (2007) Halo Wars (2008), Halo 3: ODST (2009), Halo: Reach (2010), Halo Anniversary (2011), Halo 4 (2012)) but every 3 years isn't frequent enough.

This is why I say it's the ballot or the bullet.
It's liberty or it's death.
It's freedom for everybody or freedom for nobody.
America today finds herself in a unique situation... to become involved in a bloodless revolution